In optical systems, especially those using optical fiber cables, attenuators are typically required to vary the optical output power. One requirement of such an attenuator is that the waveform of the optical output is not affected. In the art, several approaches are being followed to achieve such an optical attenuator.
An example of one such approach is described by Richard A. Soref in SPIE, Vol. 176, Guided Wave Optical Systems and Devices II, (1979), pages 124-132, in an article entitled "Fibre-Optic Switching With Liquid Crystals." In accordance with this approach, attenuation is achieved by optical switching, that is, the optical output power is varied by diverting part or all of the optical output elsewhere. In this example, optical switching is based on total or partial optical reflection by a liquid crystal layer in response to electrical commands. Thus, by varying the amount of reflection, "attenuation" of the optical output is achieved. FIG. 1 illustrates this approach.
Still another approach is described by Wagner and Chang in "Electrically Controlled Optical Switch For Multimode Fiber Applications," Applied Optics, Vol. 19, No. 17, September 1980, pages 2921-2925. FIG. 2 illustrates this approach. The optical switch illustrated divides a light beam into two orthogonal polarizations, which are separated into different optical patterns by selective reflection; it then switches both polarizations simultaneously by rotating the planes of polarization and then recombines them at one of two possible output ports by selective reflection. By switching the optical output between two points, which, incidentally, are 90.degree. apart, the optical output power in any one port varies from nearly nil to nearly 100 percent. Hence, an attenuation effect is achieved.
These two approaches are typical of the optical attenuators, or switches, in schemes involving partial or total optical reflection. These schemes have several disadvantages. Because they rely on reflections, reflective surfaces must be fabricated. This involves precisely ground and/or plated surfaces at precise angles. This presents a disadvantage in fabrication. Furthermore, because of the reflections, a disadvantage of high insertion loss results from these reflective switching schemes. Still another disadvantage is the amount of crosstalk in the switch due to the polarization impurity caused by the reflections. In other words, the separation by switching is degraded by the presence of crosstalk.
The optical switch in accordance with the preferred embodiment of the present invention minimizes these disadvantages in reflective switching attenuators. By not relying on reflective surfaces to accomplish optical switching in the attenuator, lower insertion losses are achieved. Furthermore, crosstalk separation of greater than 30 dB is realized because of better polarization purity with the novel switch.
The optical switch in accordance with the present invention displaces a light beam according to the beam's polarization. This displacement takes place in a high birefringent material, such as calcite. It then selectively rotates the polarization of the displaced beam by passing the beam through a rotator, such as a liquid crystal cell. Then, depending on the phase of the displaced beam, the beam is recombined at one port or is further displaced in accordance with its polarization. In this manner, an effective optical switch by beam displacement is realized.